Along with miniaturization of wireless communication devices and capacity enlargement of wireless communication, internet of things (IoT) devices that each incorporate a sensor device to sense various pieces of information in a living environment and that each transmit the sensed pieces of information to a server become popular.
Usually, the IoT devices each implement therein a sensing device and a load circuit such as an integrated circuit (IC) for processing sensed data. Since power supply is desired for driving the load circuit, a battery is mounted in each of the IoT devices. In order to cause each of the IoT devices to be driven for a long period of time, it is desirable to replace a battery. However, in a case where the number of installed IoT devices is large, a considerable cost is produced for replacing batteries.
As a power source that drives the load circuit and that is different from the battery, there is an energy harvesting element such as photovoltaic power generation, which converts environmental energy into electric energy. As low power consumption of the load circuit is accelerated, an increase in power generation performance of the energy harvesting element enables an IoT device to be realized, which uses, as a power source for the load circuit, the energy harvesting element in place of the battery.
On the other hand, since being greatly influenced by a change in an environment, a power generation amount based on the energy harvesting element becomes unstable. A power supply controller for stably supplying power to the load circuit is desired for reliability improvement of the IoT device. In, for example, Japanese Laid-open Patent Publication No. 2016-146156, there is disclosed a technique in which two reset ICs to operate based on different threshold voltages are provided and in which hysteresis operations are performed at a time of an increase in a power-supply voltage and at a time of a decrease therein, thereby stabilizing power supply to the load circuit. In addition, in “product FAQs”, FAQ:VD_0005_1.0, [searched on May 19, 2016], the Internet <URL:http://datasheet.sii-ic.com/pub/ic/speedfaq/jpn/power/vd/FVD5.PDF>, there is disclosed a technique for adding a resistance circuit to the outside of a reset IC, thereby widening a hysteresis. By realizing an adequate hysteresis operation, the power supply controller avoids troublesome occurrences of a connection and disconnection to and from the load circuit for a fluctuation of a value of a voltage supplied by the energy harvesting element. As a result, it is possible for the power supply controller to stabilize power supply from the energy harvesting element to the load circuit. A technique of the related art is disclosed in Japanese Laid-open Patent Publication No. 08-018010.